More of a conundrum to me has always been 'when does night start
Easy, when the sun is 12½ arc minutes below the horizon (I think). Same for sunrise
So theoretically you could be illegally logging if you are at 30,000' and log "night" as the same time as night starts on the ground. I should imagine it's a £30,000 fine, confiscation of the aircraft, and banned from flying for life
Actually, a "guestimate" is good enough for me.